The invention relates to an arrangement for lengthening sensor lines in an optoelectronic camera with the aid of a number of CCD line scanners. (linear CCD arrays)
CCD line scanners (charge-coupled devices) that are now on the market have up to 5000 pixels. Remote sensing of the earth, however, requires up to several tens of thousands of pixels in one line. For a number of technical reasons, CCD line scanners are not manufactured in such a way that they can be placed one after the other without gaps between them.
To split up light behind a projection lens or lens system of an optoelectronic, camera, German Patent Disclosure DE 36 18 400 A1, in its technically simplest version, reflects approximately half of the light available behind the lens system of the optoelectronic camera to the side, by means of a semitransparent or half-mirror-coated optical beam splitter in the form of a mirror or prism. A first long line module formed of a number of CCD line scanners is located behind the beam splitterxe2x80x94in terms of the direction of the optical axisxe2x80x94in the primary image plane of the projection lens system. A second long line module, also made of CCD line scanners, is disposed laterally of the beam splitter above the primary image plane of the projection lens system, upon a rotation of 90xc2x0, for instance, about the longitudinal axis of the module.
Besides the above-described technically relatively simple version, DE 36 16 400 A1 also describes a number of other beam splitter configurations, but all of them yield hard, discontinuous transitions between fully reflecting and fully transparent zones of the beam splitter and are disposed above the primary image plane.
In the first described version for splitting light, each long line module receives less than about 50% of the available radiation. The other beam splitter configurations described there also imply imaging gaps and the disrupting effects of diffraction.
In German Patent Disclosure DE 38 19 828 A1 , in order to split light behind the projection lens system of an optoelectronic camera, an optical beam splitter has a number of mirrors, reflecting to the right and left in alternation, or fully mirror-coated prisms. The length of these mirrors or prisms must be chosen so as to correspond exactly to the length of a single detector line of a commercial CCD line scanner.
In order to avoid gaps and the disruptive effects of diffraction in the image makeup, the beam splitter, or the axis connecting its individual mirrors or prisms, must be disposed precisely in the primary image plane of the optoelectronic camera. In this known version as well, two long line modules are provided; either they are disposed on both sides of the beam splitter, or as in one of the embodiments in the above-described patent DE 36 16 400 A1 , one long line module is disposed in the direction of the optical axis of the camera, below the beam splitter, while the other is disposed laterally of the beam splitter, offset in height, after a rotation of 90xc2x0 about the longitudinal axis of the module.
Since in this known embodiment the lengthened line is located precisely in the beam splitter, specifically in the primary image plane, the lengthened line or the beam splitter must be projected onto each of the two long line modules, via two sets of secondary lens systems. The number of secondary lens systems in each set must match the number of individual detector lines, in the form of the CCD line scanners, in the associated long line module.
As a result, each long line module receives all the light available behind the primary projection lens system; this light is somewhat attenuated only by the transmission of the secondary lens systems. Despite the segmented beam splitter disposed in the primary image plane of the projection lens system of the camera used, the large number of secondary lens systems required, and adjusting them, are the main obstacles to effective practical use; this arrangement is therefore actually used only to an extremely limited extent, if at all.
U.S. Pat. No. 4,009,388 discloses an arrangement in which the light behind a projection lens system of the camera used is split, by a half-mirror-coated optical beam splitter, to two parallel CCD line scanners that overlap one another completely.
The two CCD line scanners are offset from one another longitudinally only by one-half of one detector element, so that the centers of the individual detector elements of one CCD line scanner come to rest precisely in the boundary regions between the detector elements of the other CCD line scanners.
The method known from U.S. Pat. No. 4,009,388 therefore does not lengthen one complete line; instead, the line is merely equipped with twice as many detector elements, thereby refining the scanning but not lengthening the lines. Moreover, this method cannot be adopted to a long line module that involves gaps.
The object of the invention is therefore to create an arrangement for lengthening sensor lines in an optoelectronic camera in which a fundamentally arbitrary number of pixels can be lined up with one another in a sensor line in such a way that all the light intensity is available on a long line made of two modules.
According to the invention, in an arrangement for lengthening sensor lines in an optoelectronic camera as generically defined by the preamble to claim 1, this is attained by the characteristics in the body of that claim.
According to the invention, unlike DE 38 19 828 A1 , the beam splitter used is not mirror-coated to the extent of 50% but rather is formed of alternatingly virtually completely transparent and completely mirror-coated segments, so that the various detector lines of the CCD line scanners, or their central regions, each receive the entire light intensity.
To avoid the effects of diffraction at the transition from the mirror-coated to the transparent regions of the optical beam splitter, this transition, according to the invention, is made gradually. A small portion of the usual length of a commercial CCD line scanner suffices for this purpose.
According to the invention, interstices between successively disposed CCD line scanners in each of the two long line modules are shortened, so that the end regions of the individual detector lines of the two long line modules overlap or coincide. The length of each overlapping or coinciding region of the CCD line scanners is chosen to be somewhat greater than the sum of the length of a transition region between a mirror-coated and transparent region of the beam splitter and the diameter of the optical beam path for a pixel in the beam splitter.
Also according to the invention, the signal from the coinciding regions of the two long line modules are added together electronically. Because of the provisions according to the invention, it is assured that all the pixels, or in other words each individual detector of the overall lengthened total line, has the entire radiant intensity available to it. The advantage over DE 36 16 400 A1 that is attained with the invention is therefore that, as a result of the aforementioned adding together of the signals, virtually all the light intensity of the available radiation behind or after the projection lens system is offered to the optoelectronic detectors in the coinciding region.
Compared with DE 38 19 828 A1 , the advantages attainable with the invention are above all that secondary lens systems are avoided in the embodiment of the invention, thus averting the attendant difficulties in adjustment and transmission losses as well. Moreover, the radiometrically adulterating effects of diffraction at the radiometrically hard transitions of the reflected mirror surfaces are averted.
Furthermore, far less stringent demands are made for precision of the optical beam splitter than in the version of DE 38 19 828 A1 , where the lengths,of the individual mirrors or prisms of the beam splitter must match accurately to a fraction of one detector element, or in other words on the order of magnitude of approximately 1 xcexcm.
The arrangement according to the invention for lengthening sensor lines in an optoelectronic camera thus represents a simplification over the arrangements used previously for that purpose, and a substantial increase in quality can moreover be attained.